Store with RestPRoxy errors in adding record

I have set up an Grid and Store which use a Model that is backed by a RestProxy. (Full code at the end of this post.) The basic actions read, delete and update are working fine, but whenever I add a record, very weird behaviour occurs. First a POST is generated to create a new instance (which is fine), but after the executing of this request another PUT request is generated. After these requests, two new records reside in the grid (and) store, one with an empty id ("") and one with the correct id given back by the response to the POST request.

PS: On a side-note: Is it just me or is there no documentation available on how the server should format responses to create, delete and update requests? Neither in ext4 or ext3 on any kind of proxy resp. store?